Boron-containing carbosilanes, boron-containing oligo or polycarbosilazanes and silicon borocarbonitride ceramics

ABSTRACT

The present invention relates to boron-containing carbosilanes, a process for their preparation, boron-containing oligo- or polycarbosilazanes, a process for their preparation and their use and silicon borocarbonitride ceramics and a process for their preparation.

The present invention relates to boron-containing carbosilanes, a process for their preparation, boron-containing oligo- or polycarbosilazanes, a process for their preparation and their use and silicon borocarbonitride ceramics and a process for their preparation.

The process for producing multinary, non-oxidic ceramics via molecular single-component precursors has achieved outstanding importance. It opens up the path to nitridic, carbidic and carbonitridic systems which cannot be prepared by conventional solid-state reactions. The products are distinguished by high purity and a homogeneous element distribution on a molecular level.

Ceramic materials consisting of Si, B, N and C demonstrate particular properties with respect to thermal stability and resistance to oxidation. They can be used industrially as bulk material, in composite materials, but also for coatings or as ceramic fibres. These amorphous boron-containing materials, compared with the boron-free (Si—C—N)-compounds, are distinguished by an increased crystallization inhibition, whereas the carbon-containing compounds have, in addition, higher decomposition temperatures than the carbon-free compounds.

According to U.S. Pat. No. 5,233,066, synthesis of the amorphous ceramics Si₃B₃N₇ and SiBN₃C from the precursor trichlorosilylaminodichloroborane (TADB) is achieved by crosslinking with ammonia or amines and subsequent pyrolysis in the gas stream. According to DE-4320785, trissilylalkylboranes of the formula B[—C₂H₄—SiCl₂X]₃ are obtained by reacting vinylsilanes of the formula CH₂═CH—SiCl₂X with BH₃.THF. Carbon-rich ceramics prepared from this single-component precursor have a (Si:B) ratio of 3:1.

In Organosilicone Chemistry III VCH, (1997), via hydroboration of vinylsilane with BH₂Cl.SMe₂, a single-component precursor for Si,B,N,C-containing ceramics, abbreviated hereafter as SiBNC, is described, which has a solid Si:B ratio of 2:1, which is also retained in the ceramics.

In the case of all previously known single-component precursors, thus, disadvantageously, the Si:B ratio is bound to the single-component precursor used and the type of crosslinking The stoichiometry of C:N ratio can only be controlled via the pyrolysis conditions and choice of the crosslinking agent (NH₃ or alkylamines).

However, during the pyrolysis, the C:N ratio is set in an uncontrolled manner by reactions which have not been explained in detail. A disadvantage of these syntheses is the low possibility of varying the C content, since extending the side chain does not inevitably lead to a higher C content in the ceramics, but to graphite sediments in the material, which adversely affect the properties.

Although Appl. Organomet. Chem. 10 (1996) 241-256 describes polymeric boron-containing carbosilanes which were obtained by reacting vinyl-containing polysilanes or polysilazanes with borane adducts of the formula BH₃.SR₂, this possibility of varying the stoichiometries by using non-single-component precursor compounds, as described in J. Am. Ceram. Soc. 355 (1990) 714-718 for example, leads to ceramics whose properties (e.g. heat stability) are significantly inferior to those produced from single-component precursors.

The object of the present invention was the provision of novel boron-containing carbosilanes which are easy to prepare and permit a high possibility of varying all constituent elements, i.e. B, C, N and Si and can be used as precursors for novel, amorphous silicon borocarbonitride ceramics. For specific applications, by matching the stoichiometry, the respective property optimum is thus to be able to be set.

It has now been found that boron-containing carbosilanes of the formula (I)

where

R¹=

where

R⁴=H, C₁-C₃-alkyl and/or phenyl,

R⁵=Cl, Br,

R⁶, R⁷=Cl, Br, H, C₁-C₃-alkyl or phenyl,

R²=R¹ or Cl, Br,

R³=R⁵,

R′=SMe₂, NMe₂H,

where

n=0 when R²=R¹,

n=1 when R²=R⁵

meet the desired profile of requirements.

From these boron-containing carbosilanes of the formula (I) may be prepared carbon-rich ceramics which are distinguished by an elevated stability to alkali metals and alkaline earth metals and their compounds and also by an outstanding resistance to oxidation. In addition, the ceramics of the invention are distinguished by high moduli of elasticity and scratch resistance.

The invention therefore relates to boron-containing carbosilanes of the formula (I)

where

R¹=

where

R⁴=H, C₁-C₃-alkyl and/or phenyl,

R⁵=Cl, Br

R⁶, R⁷=Cl, Br, H, C₁-C₃-alkyl or phenyl

R²=R¹ or Cl, Br,

R³=R⁵,

R′=SMe₂, NMe₂H

where

n=0 when R²=R¹,

n=1 when R²=R⁵.

In a preferred embodiment of the invention, in the boron-containing carbosilanes of the formula (I)

where

R⁴=CH₃,

R⁵=Cl or Me,

R⁶=Me,

R⁷=R⁵,

and when

R²=R¹,

R³is Cl,

and when R² is Cl, n=1 and R′ is SMe₂.

In a further preferred embodiment of the invention, in the boron-containing carbosilanes of the formula (I)

where

R⁴=CH₃,

R⁵, R⁶ and R⁷=Cl

and when

R²=R¹

R³ is Cl,

and when R² is Cl, n=1 and R¹ is SMe₂.

The invention additionally relates to a process for preparing the boron-containing carbosilanes of the invention, in which at least one halogenoalkenesilane of the formula (II)

where

R⁴=H, C₁-C₃-alkyl and/or phenyl,

R⁵=Cl, Br,

R⁶, R⁷=independently of one another Cl, Br, H, C₁-C₃-alkyl or phenyl,

and m=0 or 1

is reacted with boranes of the formula (III)

H_(X)BR_(3- x) ⁵-SMe₂ or H_(2-x)BR_(x) ⁵-NHMe₂

where x=2 or 1,

preferably H_(x)BR_(3-x) ⁵-SMe₂

where x=2,

in an inert gas atmosphere, for example N₂, Ar, He,

in an aprotic solvent at temperatures<20° C., preferably 0-10° C.,

in which case the ratio of halogenoalkenesilanes of the formula (II) to boranes of the formula (III) is established by x, that is to say that the compounds of the formulae (II) and (III) are used in accordance with the stoichiometry x.

All of the starting materials coming under the formulae (II) and (III) are commercially available products.

For the purposes of the invention, aprotic solvents are non-halogenated or halogenated aromatic and aliphatic hydrocarbons, preferably toluene, hexane and/or dichloromethane.

In a preferred embodiment of the process of the invention, when x =1, the aprotic solvent is halogenated hydrocarbons.

The reaction of the invention is described by way of example with reference to the following diagram:

The invention further relates to boron-containing carbosilanes of the formula (IV)

where

R¹=

where

R⁴=H, C₁-C₃-alkyl and/or phenyl,

R⁵=Cl, Br, preferably Cl

R⁶, R⁷=independently of one another Cl, Br, H, C₁-C₃-alkyl or phenyl, preferably

Cl or methyl

and

R⁸=SiR⁹

where R⁹=C₁-C₃-alkyl or Cl or

The invention additionally relates to a process for preparing boron-containing carbosilanes of the formula (IV) where R⁸=SiR⁹ where R⁹=C₁-C₃-alkyl or Cl, preferably R⁸=SiCl₃, according to which at least one boron-containing carbosilane of the formula (I), in which R²=R¹, and in which R⁶ and R⁷ independently of one another are Cl, Br, H, C₁-C₃-alkyl or phenyl, are reacted in an inert gas atmosphere with Me₃Si—NH—SiCl₃ at temperatures<70° C., preferably 20 to 40° C.

Me₃Si—NH—SiCl₃ can be prepared, for example, as described in U.S. Pat. No. 5,233,066.

The reaction of the invention is described by way of example with reference to the diagram below:

The invention additionally relates to a process for preparing boron-containing carbosilanes of the formula (I) where

according to which at least one boron-containing carbosilane of the formula (I), in which R¹=R², is reacted with Me₃Si—NH—SiMe₃ at temperatures <70° C., preferably 20 to 40° C., and subsequently thereto the resultant product is reacted either with BR⁵ ^(₃)

where R⁵=Cl, Dr, or BR⁵ ₂R¹ or with BR⁵(R¹)₂, preferably BR⁵(R¹)₂ where R⁵,R⁶,R⁷=Cl, Br.

The reaction of the invention is described by way of example with reference to the diagram below:

The invention further relates to boron-containing oligo- or polycarbosilazanes obtainable by reacting the boron-containing carbosilanes of the invention with NH₃ and/or primary or secondary C₁-C₃-alkylamines, preferably MeNH₂ and/or Me₂NH.

The invention further relates to a process for preparing the boron-containing oligo- or polycarbosilazane of the invention, according to which the boron-containing carbosilanes of the formula (I) and/or (IV) are reacted with NH₃ and/or with primary or secondary C₁-C₃-alkylamines, preferably MeNH₂ and/or Me₂NH.

The process is preferably carried out at temperatures between −70 to −40° C.

To separate off the salt formed in the reaction, such as ammonium hydrochloride or alkylaminehydrochloride, the boron-containing oligo- or polycarbosilazanes can be dissolved in a solvent, particularly preferably in THF, and the salts filtered off.

The invention further relates to silicon borocarbonitride ceramics made from the boron-containing oligo- or polycarbosilazanes of the invention.

The invention further relates to a process for preparing the silicon borocarnonitride ceramics of the invention, according to which the boron-containing oligo- or polycarbosilazane of the invention is pyrolysed in an atmosphere of ammonia or inert gas at temperatures from 25 to 2000° C., preferably 1400-1800° C.

The heating rate in the pyrolysis is preferably 1-100 K/min, particularly preferably 1-20 K/min.

The invention further relates to the use of the boron-containing oligo- or polyborocarbosilazanes of the invention for preparing ceramic powders, ceramic fibres, coatings, composite materials or mouldings.

ILLUSTRATIVE EXAMPLES Example 1

Preparation of bis[(trichlorosilyl)ethyl]chloroborane 14.6 ml (15.5 g; 0.14 mol) dimethyl sulphide-monochloroborane were added dropwise to a solution of 36 ml (45.6 g; 0.285 mol) of trichlorovinylsilane in 50 ml of toluene with vigorous stirring. The temperature was kept at 10° C. during the addition by ice cooling. The mixture was then kept at this temperature for a further 5 hours and then stirred at room temperature for a further 24 hours. After removing the solvent in vacuo, 35.3 ml (49.0 g; 0.133 mol) of liquid bis[(trichlorosilyl)ethyl]chloroborane were isolated.

Example 2

Reaction of bis[(trichlorosilyl)ethyl]chloroborane with hexamethyldisilazane

10 ml of bis[(trichlorosilyl)ethyl]chloroborane (13.88 g; 37.7 mmol) were added dropwise at 0° C. to 4.0 ml (3.04 g; 18.9 mmol) of hexamethyldisilazane with vigorous stirring. The reaction solution was then stirred for a further 1 h at 40° C. to complete the reaction and resultant trimethylchlorosilane was distilled off 12.9 g (18.9 mmol) corresponding to a quantitative yield, of bis[bis[(trichlorosilyl)ethyl]boryl]amine were obtained.

Example 3

Synthesis of [(dichloromethylsilyl)]ethyldichloroborane-dimethyl sulphide

20 ml (24.44 g; 0.173 mol) of dichloromethylvinylsilane were dissolved in 10 ml of dichloromethane and 20 ml (25.1 g; 0.173 mol) of dimethyl sulphide-dichloroborane were added dropwise at 0° C. with constant stirring. After 5 h at 0° C., the reaction mixture was heated to room temperature and, after a further 24 h, the solvent was removed under reduced pressure. The yield of [(dichloromethylsilyl)]ethyldi-chloroborane-dimethyl sulphide was quantitative.

Example 4

Reaction of [(dichloromethylsilyl)]ethyldichloroborane-dimethyl sulphide with NH₃

32.8 g (0.115 mol) of [(dichloromethylsilyl)]ethyldichloroborane-dimethyl sulphide were added dropwise to 200 ml of condensed ammonia and the reaction mixture was stirred for 1 hour at −60° C. After heating and escape of excess NH₃, the product was taken up in tetrahydrofuran and the byproduct NH₄Cl was separated off by filtration under protective gas. The solvent was then distilled off and 11.6 g of a whitish-yellow solid, a boron-containing polycarbosilazane having an Si:B ratio of 1:1, were obtained.

Example 5

Pyrolysis of the solid obtained in Example 4.

The solid from Example 4 was added to a quartz Schlenk vessel and heated in argon to 100° C., without the polymer melting in the interim, and kept at this temperature for 1 hour. A black ceramic powder was obtained. The ceramic yield of Si—B—C—N ceramic was 76% by weight. 

What is claimed is:
 1. A process for preparing a boron-containing carbosilane comprising reacting, in an inert gas atmosphere in an aprotic solvent at a temperature that is <20° C. (A) at least one halogenoalkenesilane of the formula (II)

with (B) boranes of the formula (III) H_(x)BR_(3-x) ⁵-SMe₂ or H_(2-x)BR_(x) ⁵-NHMe₂  (III) wherein the boron-containing carbosilane made with the process has the formula (I)

wherein R¹ is

R² is R¹, Cl, or Br, R³ is R⁵, R⁴ is H, a C₁-C₃-alkyl or phenyl, R⁵ is Cl or Br, R⁶, R⁷,independently of one another, are Cl, Br, H, a C₁-C₃-alkyl or phenyl, m is 0 or 1 wherein x is 2 or 1 and the ratio of halogenoalkenesilanes of the formula (II) to boranes of the formula (III) is established by x, R′ is SMe₂ or NMe₂H, n is 0 when R² is R¹, and n is 1 when R² is R⁵.
 2. Process according to claim 1, wherein when x is 1, the aprotic solvent is a halogenated hydrocarbon.
 3. A process for preparing a boron-containing carbosilane of the formula (IV)

comprising reacting (A) at least one boron-containing carbosilane with (B) Me₃Si—NH—SiCl₃ at a temperature that is <70° C. wherein the at least one boron-containing carbosilane has the formula (I)

wherein R¹ and R², independently of one another, are

R³ is R⁵ R⁴ is H, a C₁-C₃-alkyl or phenyl, R⁵ is Cl or Br, R⁶, R⁷, independently of one another, are Cl, Br, H, a C₁-C₃-alkyl or phenyl, R⁸ is SiR⁹ or Cl, R⁹ is a C₁-C₃-alkyl, R′ is SMe₂ or NMe₂H, wherein n is
 0. 4. A process for preparing a boron-containing carbosilane of the formula (IV)

comprising reacting (A) at least one boron-containing carbosilane with (B) Me₃Si—NH—SiMe₃ in an inert gas atmosphere at a temperature that is <70° C. and subsequently thereto reacting the resultant product with either BR⁵ ^(₃) , BR⁵ ₂R¹ or with BR⁵(R¹)₂, wherein the boron-containing carbosilane has the formula (I)

wherein R¹ and R², independently of one another, are

R³ is R⁵ R⁴ is H, a C₁-C₃-alkyl or phenyl, R⁵ is Cl or Br, R⁶, R⁷, independently of one another, are Cl, Br, H, a C₁-C₃-alkyl or phenyl, and R⁸ is

R′ is SMe₂ or NMe₂H, and n is
 0. 5. A process for preparing a silicon borocarbonitride ceramic, comprising pyrolising at least one boron-containing oligo- or polycarbosilazane in an atemosphere of ammonia or inert gas at a temperature that ranges from 25 to 2000° C., wherein the at least one boron-containing oligo- or polycarbosilazane is obtained by reacting (1) a boron-containing carbosilane with (2) NH₃ and/or primary or secondary C₁-C₃-alkylamines, wherein the boron-containing carbosilane has a formula (I)

wherein R¹ is selected from the group consisting of

R² is R¹, Cl, or Br, R³ is R⁵, R⁴ is H, a C₁-C₃-alkyl or phenyl, R⁵ is Cl or Br, R⁶, R⁷, independently of one another, are Cl, Br, H, a C₁-C₃-alkyl or phenyl, R′ is SMe₂ or NMe₂H, n is 0 when R² is R¹, and n is 1 when R² is R⁵.
 6. A process for preparing a boron-containing oligo- or polycarbosilazane comprising A) reacting a boron-containing carbosilane of the formula (IV)

with (B) NH₃ and/or with primary or secondary C₁-C₃-alkylamines, wherein the boron-containing oligo- or polycarbosilazane made has the formula (I)

wherein R¹ is

R² is R¹, Cl, or Br, R³is R⁵, R⁴ is H, a C₁-C₃-alkyl or phenyl, R⁵ is Cl or Br, R⁶, R⁷, independently of one another, are Cl, Br, H, a C₁-C₃-alkyl or phenyl, R⁸ is SiR⁹, Cl or

R⁹ is a C₁-C₃-alkyl, R′ is SMe₂ or NMe₂H, n is 0 when R² is R¹ n is 1 when R² is R⁵. 